An optical encoder device according to the related art includes a stationary slit plate having light transmissive silts disposed at a pitch of 360° and is likely to output a signal with much distortion, thereby reducing detecting accuracy. To solve the problem, Japanese Patent Application Publication No. 2007-218603 (JP2007-218603A) teaches a known example of an optical encoder device that provides an output signal with little distortion. In the known optical encoder device, slits provided in a stationary slit plate have the following slit pattern. The slits of the stationary slit plate have a phase difference of 1/12 with respect to the pitch P of slits provided in a movable slit plate. The light transmissive slits of the stationary slit plate are divided into units so that each unit includes slits of which number is equivalent to a multiple of four. In each unit, with respect to the first slit as a reference, the first and second slits are shifted in phase by P/12, the second and third slits are shifted in phase by P/6, and the third and fourth slits are shifted in phase by P/4.
Japanese Patent Application Publication No. 60-042626 (JP60-042626A) teaches an optical encoder in which two slits have a phase difference of ⅙ to cancel a third harmonic wave.
Japanese Patent No. 2539269 (JP2539269) teaches an optical encoder in which two slits or two slit groups have a phase difference of ⅙ or 1/10 to cancel a third harmonic wave or a fifth harmonic wave.
Japanese Patent No. 3184419 (JP3184419) describes that n orders of harmonic waves are canceled by using 2n or more slits.